Recent standards promulgated in National Electric Code (“NEC”) Article 830—Network-Powered Broadband Communications Systems, list requirements that pertain to powered Broadband Communications Networks (i.e. CATV networks). The networks are classified in terms of Low, Medium and High Power. A powered “drop” from the “tap” on the hard-line coax to the NIU (Network Interface Unit) or any other CPE (customer premise equipment) is generally considered to meet the requirements of a Low Power Network. For example, Section 830-11(c) requires that underground drops be buried at least 18″ deep and calls for mechanical protection (i.e. conduit) where the cable emerges from the ground. This requirement reportedly costs service providers well in excess of $100 to implement.
However, an exception to this requirement provides that Low Power circuits that are also equipped with a “listed fault protection device, appropriate to the network-powered broadband communications cable used” need not be buried, as long as the appropriate fault protection device is “located on the network side of the network-powered broadband communications cable being protected.” Section 830-2 of the NEC defines, very generally, what a Fault Protection Device is. The main intention is to “provide [to humans or animals] acceptable protection from electric shock.”
An existing system marketed as providing this protection comprises a two-device system with one device residing at the tap and the other at the NIU. The device works by generating a small DC offset voltage in the tap end. This voltage is passed through a known impedance at the NIU end device. An increase in impedance is seen as an open and a decrease in impedance is seen as a short on the drop. Both cases cause the tap end of the device to disconnect the power from the drop.
One drawback of this system is an inability to handle load transients placed on the network by the NIU. These transients appear as a near DC offset in current and cause the device to trip, thus disconnecting the power to the NIU. The manufacturer has attempted to overcome this problem by slowing the response of the device (increasing response time). However, as response time increases, personnel protection decreases because the energy of a fault transient is directly proportional to its period. Thus, to provide the greatest margin of safety, response times should be kept as short as possible.
Another device monitors the input impedance of the NIU. Deviations of the input impedance are interpreted as shorts or opens and cause the device to trip. However, this method does not account for the variation of the input impedance of the NIU power supply due to the variation in both the line voltage and the load placed on it by the NIU.
Further, assuming the above problem is overcome, a different device would probably be required for every type NIU to account for power supply and EMI filter differences.
Therefore, a need exists for a fault protection method and architecture that provide a fast response time and are effective for variable input impedance and type of NIU.
Furthermore, for literal compliance with NEC section 830, a need exists for a fault protection method and architecture that comprises a single device electrically and physically located on the network side of the cable being protected.